Thermal and dissolved gas effects on cavitation in a 2-D convergent-divergent nozzle flow

It is well known that the thermal effect of cavitation suppresses the development of cavity with the decreased local temperature due to the latent heat of evaporation. In the present study, to obtain the fundamental knowledge of thermal effect of cavitation, an experimental study of cavitation in a simple convergent-divergent nozzle was carried out using hydrofluoro- ether (HFE) as a working fluid. HFE was expected to reveal the thermal effect at room temperature, which enabled us to simply neglect the heat flux between the test section and outside, and to roughly keep adiabatic conditions on nozzle wall surfaces. It was found that the cavitation in HFE was a foam-like sheet cavity, which was different from a film like cavity in cold water. The temperature depression, near the leading edge of cavitation, was confirmed especially in more developed cavitation cases, revealing the existence of thermal effect of cavitation. However, in lower velocity cases, the measured cavity length was apparently longer in HFE despite of the expected thermal effect. The measured pressures inside the cavity were found to be apparently larger than the vapor pressure, indicating the possible separation of dissolved air into the cavity. Then, the cavitation number based on the measured cavity pressure instead of vapor pressure was proposed, and by using the proposed cavitation number, the thermal effect was clearly confirmed in terms of the cavity development. Frequency characteristics of measured pressure fluctuations were qualitatively similar regardless of the species of working fluid.